1. Field of the Invention
This invention is related to multiple satellite communication systems. In particular, it is related to traffic control in a satellite network for optimizing link allocation.
2. Discussion of Related Art
Terrestrial cellular communication systems are well known. Multiple Satellite communication systems complement terrestrial cellular communication systems to augment traffic handling capacity and service areas where wire or cellular networks have not reached. Satellite systems came into existence in response to the need for efficient and economical mobile communications. In general, the satellites act as a transponder, or "bent pipe", receiving ground based transmissions from one location and beaming the repeated transmission back down to another location after amplification and frequency shifting, as discussed in U.S. Pat. No. 5,448,623, incorporated herein by reference in its entirety.
Terrestrial cellular communications are organized into "cells" to increase the system capacity in view of a limited number of frequencies available. For Code Division Multiple Access (CDMA) applications, a smaller cell size reduces the amount of self-interference, that is, interference generated by users of the same system. Terrestrial cellular systems use link diversity "to provide two or more inputs at the mobile reception unit so that fading phenomena among those inputs are uncorrelated", as discussed by William C. Y. Lee, "Mobile Communications Design Fundamentals" incorporated herein by reference in its entirety.
A hand-off is a feature of a terrestrial cellular system which switches a user to a new cell without interrupting the call or alerting the user. Path diversity, that is, allocation of different transmission paths between caller and base station, is generally used as part of the hand-off process. Pilot signals from the base stations are used to establish a reference clock for the reception of CDMA signals. The strength of the pilot signal determines the cell radius. The decisions on cell size are driven by the amount of power needed for mobiles on the link that returns the signal to the base station(s).
In contrast to the terrestrial cellular systems, in multiple satellite systems, a limiting factor to the size of an area to be served is the amount of satellite power needed for the forward (satellite to user) link. This is different from terrestrial cellular systems where the limiting factor for area coverage is the power of mobile subscribers in the return links. In order to use low cost satellites, there are operational limits to the amount of instantaneous power that a satellite and/or its associated beam can provide. In addition, international specifications detailing satellite emissions, such as ITU RR 2566, limit the amount of power flux density received at a point on the earth from a satellite. Like in terrestrial cellular system, satellite based CDMA systems have self-interference limits constraining system capacity. Both terrestrial and satellite systems use path diversity to improve link availability. Unlike terrestrial cellular systems, the effects of self-interference on the capacity of satellite-based mobile communication systems employing CDMA is not improved by subdividing the service area in a plurality of cells. This is because self-interference in satellite systems has different causes. The forward link using a channel in the satellite system is "orthogonal". This means that other users in that link on the same channel do not contribute to self-interference. Instead, self-interference is caused by signals from other channels, or other satellites, or other gateways sharing the same channel (i.e., frequency spectrum) and from satellite multi-beam antenna "side-lobes". Other differences include a larger link delay caused by the orbiting satellite path. This time delay makes power control by a gateway using dynamic traffic allocation less effective.
The optimization of traffic and power allocation in view of the above described considerations has been discussed in the prior art. Authors such as Gagliardi, "Satellite Communications" and B. R. Rojcic, R. L. Pickholtz, and L. B. Milsteain, "Performance of DS-CDMA with imperfect power control operating over a Low Earth Orbiting Satellite Link", IEEE Journal on Selected Areas in Communications, provide simplified analytic models. A limitation of these models is the use of simplifying assumptions such as an ideal "isoflux" antenna, and an equal amount of power in all beams, which preclude their use for power allocation optimization. An isoflux antenna provides gain sufficient to maintain received power as a constant. The fading model uses a weighted combination of Rician and Rayleigh fading. The models, however, do not account for the differences that occur with satellite elevation.
Another unsolved problem in the prior art is the optimum balance between using more diversity to improve link availability in view of satellite power usage. The orthogonal signals in the forward link are most efficiently combined coherently. A coherent combiner adds signal amplitudes. The combining efficiency is best when the signal-to-noise ratio of the received signals are balanced. Unfortunately, this may occur when one of the diversity paths is using a low elevation angle satellite. At low elevation angles an excessive amount of power may be required to support the desired signal-to-noise ratio.
The capability to make diversity decisions involving many satellites for gateways of many service areas did not exist previously. In the past, decisions were made based on information local to a service area or signal environment. While these decisions did consider operational constraints in the satellite system, satellite availability decisions were only considered implicitly as part of the operational strategies.
Also, simulations of satellites systems with explicit decision rules for picking satellite links for diversity were also employed in the past. The disadvantage of explicit decisions is that they must be reconsidered in light of other system constraints such as satellite thermal and battery limits.
The application of power allocation to the decision making process of a User Terminal Power Allocation System has been considered in the past. The desired solution is for a power allocation that considers the power limitations of a satellite system and the diversity needs of the user terminals. Prior art tools to arrive at the desired solution is Convex Programming and Artificial Intelligence search methods such as Genetic Programming.
Linear Programming methods were generally used in the prior art to solve network routing problems. In these, the Multiple Satellite System can be considered a network with the primary path as the "highest elevation satellite". The highest elevation satellite has the least "cost" with respect to instantaneous power usage. Overflow paths can be considered as the next highest elevation satellite. Unfortunately, this model breaks down when considering diversity and limits on battery Depth of Discharge. The power allocation problem is to find single path or multiple paths depending upon the desired level of diversity. This model also does not apply when satellites are given priority because of battery or thermal system conditions.
The Linear Programming method of the prior art is a technique for optimizing an objective with a number of constraints with a linear problem structure. "Barrier" methods use an interior search approach. These methods are extremely effective for large problems with large numbers of variables. Tight bounds can be defined for the computational time of a problem.
The Convex Programming method of the prior art is a technique for reducing the computational burden for solving nonlinear, but convex problems. The convergence and rate of convergence for the barrier methods are well established in the context of Convex Programming methods. Convex programs can be solved at the same computational rate as Linear Problems, however, no existing large scale systems exist. Genetic Programming, another technique of the prior art, provides solutions to non-linear problems. However, no bounds can be determined for the solution time that is typical of genetic programming procedures.
Given the limitations of the prior art as discussed above, it is a first object of this invention to provide an apparatus and method for optimizing multiple satellite system capacity using a power allocation strategy that considers requirements of spatial link diversity for user terminals, satellite power limitations and self-interference considerations. This optimization minimizes loss of traffic capacity (i.e., revenue) due to misallocation of available power and diversity.
It is another object of this invention to improve the balance between a plurality of paths between a plurality of satellites and ground grid points, for improved signal to noise ratios while maintaining satellite power usage within prescribed limits.
It is yet another object of the invention to allocate diversity paths using satellites at elevation angles that will support desired signal to noise ratios while precluding the use of excessive amounts of power.
It is yet another object of the invention to provide a method of operating a satellite system that optimizes path diversity in light of battery Depth of Discharge requirements.
It is yet another object of the invention to provide a method of operating a satellite system that optimizes power allocation and diversity derived single path or multiple paths when one or more satellites of a plurality of satellites are given priority related to battery or thermal system limitations.